1. Field of the Invention
The present invention relates to coatings for oxidizable metals, and more particularly to corrosion-resistant coatings containing intrinsically conductive polymers; to methods for using such coatings to protect oxidizable metals from corrosion; to oxidizable metal surfaces protected from corrosion by such coatings; and to compositions that may be used to form such coatings.
2. Description of Related Art
Coatings for metals are important in a number of applications, one of which is the protection of the metal from corrosion. Corrosion resistant coatings for metals are designed: (1) to form a barrier between the metal and the environment; (2) to provide cathodic protection of the metal, as with a zinc-rich primer; and/or (3) to passivate the metal, as with a heavy metal oxide of, for example, chromium or molybdate. Problems in providing barrier coatings free of pin-holes and environmental concerns about heavy metals have resulted in increased interest in protective coatings that include intrinsically conductive polymers.
Intrinsically conducting polymers (ICP's), are polymers such as polyaniline, polypyrrole and polythiophene that have poly-conjugated .pi.-electron systems and that conduct electricity in at least one valence state. It is believed that coatings for metals that contain ICP's have conductivity properties that passivate and protect the metal from corrosion; even where the coating is penetrated by pin-holes or scratches. However, limitations in the processability of ICP's along with the brittleness and lack of strength and adherence of films composed only of ICP's has limited their commercial application. See, for example, Deng, et al., J. Electrochem. Soc., 136:2152-2157, 1989; Ren and Barkey, J. Electrochem. Soc., 139:1021-1026, 1992; Wessling, Adv. Mater., 6:226-228, 1994; and Lu, et al., Synth. Metals, 71:2163-2166, 1995.
ICP's can be incorporated into corrosion resistant coatings by applying a film that includes a dispersion or solution of the polymer, or by polymerization of an ICP monomer into an ICP in situ onto the surface to be protected, by either chemical or electrochemical means.
A film containing an ICP in polymer form can be applied to a surface in the form of a paint or liquid coating formulation. The carrier solvents can then be evaporated leaving the ICP in the form of a film on the surface to be protected. Use of an ICP in polymer form has the advantage of permitting the production of the ICP under conditions that are optimum for the desired molecular weight and conductivity properties, but such method also requires the use of a solvent and the evaporation of the solvent can cause air pollution problems.
Alternative methods of application of ICP's in polymer form can be electrodeposition (See, e.g., U.S. Pat. Nos. 5,543,084 and 5,556,518), or incorporation of ICP's into formulated coatings, (See, e.g., U.S. Pat. Nos. 5,494,609, 5,290,483, 5,006,278, 5,532,025, JP 5003138 A, JP 6045196 A and JP 6045195 A), or paints, (See, e.g., U.S. Pat. No. 5,441,772 and PCT Publ. No. WO93/14166).
It has been reported that anti-corrosion films containing polyaniline having improved thermal and pH stability against de-doping can be produced by the use of so-called, "double-strand" polyaniline. (See, e.g., Sun, et al., Mat. Res. Soc. Symp. Proc., 328, 1993; Racicot, et al., Synth. Metals, 85:1263-1264, 1997; U.S. Pat. No. 5,489,400 to Liu et al.; Racicot et al., SPIE Reprint: Optical and photonic applications of electroactive and conducting polymers, 2528:251-258, 1995; Racicot, et al., Mat. Res. Soc. Symp. Proc., 413:529, 1996, and Racicot et al., Corrosion protection comparison of a chromate conversion coating to a novel conductive polymer coating on aluminum alloys, Paper No. 531, Corrosion 97, NACE International, 1997). Such polyanilines are produced by chemical polymerization of aniline in the presence of a multifunctional polymeric acid ("polyacid") such as polystyrenesulfonic acid or poly(methylacrylate-co-acrylic acid) to form a complex of the polyaniline and the polymeric acid. It is believed that the tight binding of the large multi-anionic polymeric acid with the amine groups of the polyaniline enhanced the stability of the doped form of the polyaniline to de-doping by high temperatures and basic pH values. The group reported promising use of a polyaniline/poly(methylacrylate-co-acrylic acid) complex as an anti-corrosion coating for aluminum when the complex was dissolved in ethyl acetate and applied to cleaned and polished aluminum alloys AA7075 and AA2024 as a liquid. Upon evaporation of the solvent, the complex formed a film that was reported to have a corrosion current density that was 2 to 3 orders of magnitude lower than a conventional anodized aluminum oxide film. None of these references, however, reported film formation by in situ electrochemical polymerization. In fact, U.S. Pat. No. 5,489,400 contrasted the process of electrochemical polymerization as being quite different from those of the "template guided" chemical polymerization preferred in that invention. It was also stated that the molecular complexes made by chemical polymerization are also quite different from those made by electrochemical polymerization.
Formation of polyaniline by electropolymerization from aqueous solutions that also contained a polyacid has been reported by Hyodo et al., in Electrochemical Acta, 36:87-91, 1991, and by Hwang and Yang, in Synthetic Metals, 29:E271-E276, 1989. Neither of these references deposited ICP films on aluminum, however, and all electrooxidation was carried out at electrical potentials under 0.75 volts. In fact, Hyodo et al. stated that polyaniline degradation was suppressed by limiting cell potential to 0.75 volts.
In situ chemical polymerization of ICP's into films on metal surfaces has been reported for the construction of electrical batteries and capacitors. Although the art of producing batteries and capacitors is not closely related to the formation of corrosion-resistant coatings, it has been found that such solid state devices can be formed from a layer of ICP's deposited on a valve action metal, such as aluminum or tantalum, with an interlayer of a dielectric, such as the metal oxide. These techniques first require the formation of a metal oxide layer on the surface of the metal and then coat the oxide with the ICP. The ICP can be applied in polymer form as previously described, or it may be polymerized in situ (i.e., polymerized on the surface of the oxide to form an ICP film). In situ polymerization can be carried out by applying to the metal oxide either a monomer polymerizable into an ICP (ICP monomer) followed by contact with a separate solution of a chemical oxidant, (See, e.g., U.S. Pat. No. 5,567,209), or first applying a chemical oxidant followed by a solution containing an ICP monomer, (U.S. Pat. No. 4,780,796). Other methods form a first ICP layer by chemical polymerization and then apply an additional layer of a second ICP by electrochemical polymerization. See, e.g., EP 591035, U.S. Pat. No. 4,780,796, JP 6045200 A and JP 6045199 A. The advantage cited for such two-stage application of the ICP layer is that the insulating properties of the metal oxide layer make it impossible, or, at least, difficult, to deposit the ICP electrochemically, (See, e.g., U.S. Pat. No. 4,780,796).
Ohsawa et al. (U.S. Pat. No. 4,948,685 and U.S. Pat. No. 4,999,263), however, used electrochemical polymerization of polyaniline to form sheet-shaped electrodes and demonstrated the electrochemical polymerization of polyaniline onto nickel, stainless steel and substantially pure, surface-roughened aluminum at electrode potentials of up to 1.2 volts versus a saturated calomel electrode (V/SCE) in aqueous solutions of acids having pK.sub.a values between -2.5 and +2.5. Sulfuric and p-toluenesulfonic acids were successfully used for the electropolymerization, but no polyaniline film was produced on aluminum when acids having pK.sub.a values of about 3 (nitric), 3.1 (perchloric), 3.2 (hydrofluoric) 4.0 (hydrochloric) and 4.9 (tetrafluoroboric) were used. It was thought that the surface-roughening procedure facilitated the formation of the ICP film even when electrochemical polymerization was used after an oxide layer had formed on the metal.
Application of an ICP coating by electrochemical means to a metal surface to be protected from corrosion would offer several advantages over the application of the coating as a paint. For example, the metal is usually highly conductive and makes a good electrode, anodic electropolymerization of ICP monomers is well known and does not require the use of expensive chemical oxidants and electropolymerization of ICP monomers does not produce by-product salts. In addition, electrochemical polymerization provides the immersed metal surface with throrough coverage by the ICP film. However, attempts to form corrosion-resistant ICP coatings on oxidizable metals by direct electropolymerization have encountered problems. Many of these problems center around how to carry out the electropolymerization while maintaining a conductive form of the ICP. Such problems include the formation of an unwanted oxide layer that prevented or hindered the synthesis of an ICP film at voltages that were sufficiently low to avoid over-oxidation of the ICP; the synthesis of weak, powdery films, in some cases due to over-oxidation; and the inability to incorporate other components into the film to improve its strength and adherence.
DeBerry, J. Electrochem. Soc., 132(5):1022-1026, 1985, deposited polyaniline coatings on ferritic stainless steels by electropolymerization. The metal substrate to be coated served as the anode, or working electrode, in an electrochemical cell containing 1.0 M aniline in aqueous pH 1.0 perchloric acid solution. The polyaniline coating was formed by cycling the working electrode potential between about -0.2 and +1.1 volts versus a saturated calomel electrode. It was reported that the polyaniline coatings were deposited over a passive metal oxide film, but could undergo electron transfer with the metals, and it was stated that the possibility that the polyaniline may penetrate the oxide film could not be ruled out. Polyaniline coated stainless steels, even with scratches through the coatings, remained passive for long periods of time in acid solutions which would normally attack the steels at high rates of corrosion, but the stability of the coatings was inconsistent and strongly dependent on the methods used before, during and after electropolymerization.
Later work by Geskin, J. Chem. Phys., 89:1221-1226, 1992, reported the deposition of polyaniline onto nickel to minimize corrosion. Degreased nickel sheets and nickel sputtered onto insulating substrates were used as working electrodes in an electrochemical bath containing 1.0 M sulfuric acid in aqueous solution with 0.4 M aniline. Initial immersion of the nickel into the bath at a potential of 0.6 volts and then cycling through the normal aniline oxidation range of from about -0.2-+1.0 volts deposited polyaniline films on the metal, but potentials exceeding 1.5 V caused oxygen evolution at the electrode surface and prevented coating with the polymer and also led to over-oxidation of polyaniline. The best coatings were obtained at 1.4 V.
More recently, Beck, Metalloberflaeche, 46(4):177-182, 1992, surveyed literature reports of the electropolymerization of polypyrrole and polythiophene to give highly adhesive films that provided corrosion protection to metals such as iron, aluminum and copper. Polypyrrole coatings on such base metals that have a tendency for active anodic dissolution were reported in special electrolytes such as aqueous solutions of 0.1 M potassium nitrate, 0.1 M oxalic acid, phosphate buffer and 0.1 M tetraethylammonium paratoluenesulfonate in propylene carbonate. Cell potentials of between 0.5-0.9 volts were indicated. Polybisthiophene coatings were deposited from a solution of 0.1 M potassium nitrate in 60 vol. % acetonitrile. Anodic co-deposition of polypyrrole and such powdery solid pigments as titanium dioxide, carbon black and the like was discussed. The possibility of applying such ICP's to metals as a replacement for galvanizing or phosphatizing as base coats for conventional metal coatings was also mentioned.
However, at about the same time, a separate group attempted to coat stainless steel and mild steel by the electropolymerization of aniline, pyrrole, furan and thiophene. According to Troch-Nagels, et al., in J. of Appl. Electrochem., 22:756-764, 1992, attempts to coat mild steel with polyaniline in neutral and basic solutions of water and methanol at between 0.8-1.5 V/SCE yielded non-conductive brown films. In 20 vol. % methanol and 0.13 M sulfuric acid, no films were produced below 0.8 V/SCE and poor quality, black and powdery films were produced when the potential was above 1.4 V/SCE. Polyaniline films also were reported to have been produced from 0.3 M aniline solutions in 0.1 M nitric acid with electrode potentials cycling between about -0.5-+1.5 V/SCE, but the films were powdery and brittle. While better films were reported to have been made on mild steel from 0.5 M pyrrole in 0.08 M sodium sulfate at 0.8-1.4 V/SCE, adhesion was reported to have been poor.
No suitable films were formed from either furan or thiophene. Troch-Nagels, et al. concluded that polyaniline films on mild steel did not meet the requirements for industrialization because they were brittle, did not allow anaphoretic painting and did not improve corrosion resistance. Polypyrrole films were discovered to be slightly better, but their adhesion was found to be poor and the films were brittle. Aluminum was not included in the work.
More success was reported by Li, et al., Beijing Keji Daxue Kuebao, 13(4):367-372, 1991, for the electropolymerization of polyaniline films onto stainless steel and carbon steel from solutions of aniline in sulfuric acid. It was reported that the films lowered the corrosion current densities by about three orders of magnitude, but the films would dissolve and fall off because of over-oxidation when the polymerization potential arose to 0.65 V in NaCl or 0.9 V in sulfuric acid.
Additional questions are associated with the use of ICP's in corrosion resistant films. For example, there is debate whether it matters if the ICP is in the conductive or the nonconductive form, or, whether the film itself is conductive or nonconductive. Although much of the work reported in this area suggests that the conductive form of the ICP is more effective for corrosion resistance (See, e.g., Mattson, "The Synthesis of Conducting Polymers for Corrosion Prevention", Final Report on NASA Contract NASA-NGT-60002, N89-14159, 1988; Thompson, et al., "Corrosion-Protective Coatings from Electrically Conductive Polymers", Proceedings from Technology 2001, San Jose, Calif., 1991; U.S. Pat. No. 4,855,361; and U.S. Pat. No. 5,006,278), other reports maintain that the nonconductive form is superior (See, e.g., U.S. Pat. No. 5,441,772). Thus, it is not obvious from the prior art which form of the ICP is more effective for corrosion resistant films and coatings.
A further problem that has been found to hinder the commercialization of doped ICP coatings has been the tendency of the dopant to leach out of the film during exposure to water. Such loss of the dopant will, in time, leave the film will little or no electrical conductivity. This problem has been encountered during exposure to rain and also during processing, such as, for example, during hot water sealing of anodized aluminum surfaces.
In view of these efforts, it is clear that despite the potential advantages offered by the polymerization of ICP monomers directly onto metal surfaces to form on such surfaces corrosion resistant coatings comprising ICP's, problems remain which have restrained the commercial use of such films to protect oxidizable metals such as aluminum. Accordingly, it would be desirable to provide methods for forming ICP-containing films on aluminum that improve its resistance to pitting-type corrosion, especially in salt environments. It would also be desirable to provide methods that were capable of forming such films despite the presence of aluminum oxide films on the aluminum surface and without tendency toward over-oxidizing the ICP during formation. It would also be desirable to provide methods to produce such ICP-containing films wherein the ICP is present in the doped, conductive form and will remain in that form without de-doping during hot water sealing, or environmental exposure. Moreover, it would be desirable to provide methods to protect aluminum from corrosion by the application of such a coating as just described. It would also be desirable to provide an aluminum surface that was protected from corrosion by a coating such as described. Finally, it would be desirable to provide a composition from which such a coating could be formed.